If you drive through Maryland, the state may be using an automated reader to photograph your license plate — and storing your movements away for future use. Maryland is not alone. ACLU offices in 38 states are looking into how the government is using license-plate readers across the country — and what it is doing with the data. The ACLU is already calling the license-plate readers “the next big thing in government tracking.”

There are some uses of automatic license-plate readers that most people would agree are relatively unobjectionable — looking for cars that fled crime scenes or have been stolen, for example. The real problem is that when the government stores that information, it is not trying to solve an ongoing crime — it is building a database. These databases can quickly fill up with all sorts of details about how people lead their lives. By piecing together the locations of a particular license plate over time, the government may be able to determine if someone goes to church, synagogue or mosque regularly; whether they go to meetings of a particular political group; whether they participate in protests; or even if they are having an affair.

It’s hard to know how widespread the technology is, but to give one example, Los Angeles County alone is using hundreds of license-plate readers. According to LA Weekly, which got its numbers in part through public-records requests, Los Angeles police have recorded more than 160 million data points about the movements of millions of drivers.

It would be troubling enough if the license-plate data stayed instate, but it doesn’t. Maryland, for example, shares its records with a “fusion center” — an antiterrorism office that is run jointly by federal, state and local governments. That means that the federal government can combine data from different states and track people’s movements across the entire country.

The federal government is also using license-plate readers. The U.S. Drug Enforcement Administration, which has been trying to get permission to use the readers in Utah, stated publicly that it is already operating scanners along drug-trafficking corridors in Texas and California. The federal government is also making money available to states to acquire license-plate readers. The ACLU of Massachusetts has filed a federal Freedom of Information Act request to learn more about how the federal government is using and funding license-plate readers.

But are scanners a violation of privacy? There used to be general agreement that activities like driving, which occur on public streets, are not private — and that people have no right to complain when their movements are being tracked. But the rise of highly invasive technology and databases is changing that. As one federal appeals court put it in an influential ruling involving the police planting GPS devices on people’s cars, these high-tech instruments allow the government to put together a “mosaic” of how people live their lives — a massive privacy violation.

Bottom line: license-plate reading should not be done in secret. The public has a right to know what kind of monitoring the government is doing, and there should be a public discussion of the appropriate trade-offs between law enforcement and privacy rights. If the ACLU offices get the information they want about how the federal and state governments are using license-plate readers, that discussion can begin.